Most casting alloy development effort for several decades for alloys useful in the petrochemical and heat treatment applications has been directed toward improving the hot strength of such alloys. Weldability is also extremely important because large castings are welded together for original installations. Further, the industry has also shown great interest in alloys suitable for repair welding after extended periods of service but most current alloys have shown marked tendency to embrittle or lose cold ductility after service periods making repair welding impracticable.
The only practical weldable cast heat resistant alloys have been based upon various combinations of nickel, iron and chromium. Those alloys have often been enhanced by further additions of fractions of a percent up to several percent of one or more elements from the group, cobalt, tungsten, molybdenum, niobium, tantalum, titanium and zirconium and are generally deoxidized by approximately a percent or less each of silicon, manganese and, sometimes, aluminum. Those alloys derive their hot strengths partly by solid solution hardening and partly by formation of precipitated carbides. Such alloys, developed over a period of several decades and containing about 0.45% to 0.55% carbon, have had high hot strengths but have been found to be virtually unweldable by ordinary methods. Contrariwise, those alloys of about 0.40% or less carbon have been weldable but of generally much lower hot strengths than the higher carbon alloys. Thus, there remains a great demand for alloys having the weldability of the 0.40% or lower carbon content alloys but with the hot strengths achieved by the 0.45% to 0.55% carbon alloys and especially for such alloys that are capable of long term service in the 1800.degree. F. to 2100.degree. F. temperature range. This situation is illustrated by the data in Table 1 and Table 2 below which presents the published hot strengths of typical commercial alloys from both the high carbon and low carbon groups. Alloys above the dashed lines in each table are those that nominally contain about 0.45% or more carbon by weight, while those below the dashed lines are those that contain nominally 0.40% or less carbon.
TABLE 1 ______________________________________ COMPOSITION OF CAST HEAT RESISTANT ALLOYS, WT. % Alloy Designation C Ni Cr Fe Others ______________________________________ Supertherm .50 35 28 15.5 15Co, 5W HP Microalloyed .45 35 25 37 .5W, .25Nb, .10Ti H110 .55 33 30 29 4.5W, .5Nb, .10Ti NA22H .45 48 28 16 5W HP50W2 .50 35 25 32 5W, .5Zr HP55 .55 35 25 37 -- More 2 .20 50 33 0 16W, 1Al HP-Nb .40 35 25 36 1.5Nb IN519 .35 24 24 48 1.5Nb IN625 .20 63 22 2 9Mo, 4Nb + Ta Hastelloy X .10 48 22 18.5 9Mo, 1.5Co, .6W CR30A .06 51 30 15 2Mo, .2Ti, .14Al, .02Zr ______________________________________
TABLE 2 ______________________________________ 10,000-HOUR RUPTURE STRESS AT VARIOUS TEMPERATURES, PSI Alloy Designation 1800.degree. F. 1900.degree. F. 2000.degree. F. 2100.degree. F. ______________________________________ Supertherm 3800 2400 1300 650 HP Microalloyed 3000 1900 1100 500 H110 2700 1750 900 450 NA22H 2500 1450 830 450 HP50W2 2500 1400 750 400 HP55 2500 1350 700 400 More 2 2750 1650 1000 600 HP-Nb 2700 1600 830 450 IN519 2300 1300 700 380 IN625 2250 1100 650 400 Hastelloy X 1250 660 350 200 CR30A 1900 900 420 320 ______________________________________
While More 2 alloy is presently formulated to contain less than 0.40% C, it is characterized by both high hot strength and lack of weldability comparable to the high carbon group of alloys. Thus, the sought after alloy is one that has hot strengths comparable to More 2 alloy or higher combined with the weldability of the other low-carbon alloys.